Physical Properties of Rhodium

Physical properties of Rhodium are related with its melat nature. Rhodium is a white metal resembling aluminium in appearance. Its density has been variously given in values ranging from 11 to 12.1. Its mean specific heat is 0.058, and its coefficient of linear expansion with rise of temperature is 0.04058.

Rhodium melts at 1907° C. and boils at about, 2500° C. Physical properties of Rhodium are different from those of similar metals. It is less volatile than platinum, and when alloyed with that metal not only stiffens it, but, unlike iridium, reduces its volatility at all temperatures above 900° C. It has been suggested, therefore, that a useful alloy for best quality crucibles would consist of platinum 95 to 97 per cent., and rhodium 3 to 5 per cent., and containing no other detectable impurities. Below 900° C. the presence of rhodium appears to exert a negligible effect. When cooled to -80° C. rhodium appears to undergo a molecular transformation of some kind, analogous to that evidenced by copper. At this temperature the electrical resistance is considerably below the calculated value. The most intense lines in the spectrum of rhodium are as follow:

When rhodium is melted in the presence of charcoal some 7 per cent, of the carbon is dissolved, only to be thrown out again in the form of graphite on cooling. When heated in air the surface of the metal becomes slightly tarnished or oxidised.